Derivative couplings and analytic gradients for diabatic states, with an implementation for Boys-localized configuration-interaction singles

We demonstrate that Boys-localized diabatic states do indeed exhibit small derivative couplings, as is required of quasidiabatic states. In doing so, we present a general formalism for calculating derivative couplings and analytic gradients for diabatic states. We then develop additional equations specific to the case of Boys-localized configuration-interaction singles (CIS)—in particular, the analytic gradient of the CIS dipole matrix—and we validate our implementation against finite-difference results. In a forthcoming paper, we will publish additional algorithmic and computational details and apply our method to the Closs energy-transfer systems as a further test of the validity of Boys-localized diabatic states. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in S. Fatehi et al., J. Chem. Phys. 139, 124112 (2013) and may be found at http://dx.doi.org/10.1063/1.4820485.